Optical systems are now a key part of telecommunications due to their ability to handle enormous amounts of information. Typical systems include a light source, such as a laser, an optical fiber to receive the light from the source and transmit it to some location, and a photodetector to convert the incoming light into an electrical signal. Typically, information is impressed on the light by means of a modulator which converts the light to a series of optical pulses. One example of such a modulator is an electroabsorption modulator (EAM) which applies an electrical bias to one or more semiconductor layers including quantum wells to change the absorption of the wells and thereby produce a series of amplitude modulated pulses corresponding to the information from the electrical signal. One of the advantages of such a device is that it can be integrated with the laser source.
Optimal performance of an EAM preferably includes good chirp performance, adequate extinction ratio, low operating bias and drive voltages, low insertion loss, high optical power handling capability, and a fast switching time. Optimizing for one of these parameters usually degrades the other parameters. Of particular importance for high speed transmission (2.5 Gbits/sec or greater) is the dynamic, or “transient” chirp, which is the frequency shift that accompanies changes in transmitted optical power on the rising and falling edges of an optical pulse.
A common metric to describe the chirp performance is the small signal alpha parameter, α, which expresses the relationship between the phase shift of the optical signal and the derivative of the transmitted power. For light transmitted through an EAM, this can be recast in terms of the ratio of the differential change in modal index of refraction to the differential change in modal absorption. In this form, the alpha parameter, α (V,λ), is given by:                               α          ⁡                      (                          V              ,              λ                        )                          =                                                            δ                ⁡                                  [                                      Δ                    ⁢                                                                                   ⁢                                          n                      ⁡                                              (                                                  V                          ,                          λ                                                )                                                                              ]                                                            δ                ⁡                                  (                  V                  )                                                                                    δ                ⁡                                  [                                      Δ                    ⁢                                                                                   ⁢                                          a                      ⁡                                              (                                                  V                          ,                          λ                                                )                                                                              ]                                                            δ                ⁡                                  (                  V                  )                                                              ×                                    4              ⁢              π                        λ                                              (        1        )            where V is the applied reverse bias, a is the modal absorption, n is the modal index of refraction, and λ is the wavelength of the modulated light. It is a function of both the applied voltage and the transmission signal wavelength.
For high speed transmission systems it is often highly desirable to launch amplitude modulated signals with a low or negative dynamic chirp, as this can increase the system power penalty margin or allow longer span distances to be realized.
Conventional EAMs comprise quantum wells in which the active layer is formed from rectangular quantum well structures (i.e., high energy barriers on either side of a single low energy well. However, significant performance trade-offs have to be made in the design of multi-quantum well structures, and especially for EAMs that are integrated with lasers. For example, it is known that the alpha parameter may be reduced by employing shallow quantum wells or wide quantum wells with specifically tailored strain. However, use of shallow wells tends to increase on-state (i.e. high transmission power) insertion loss and reduce the extinction ratio over the available bias range for only a small reduction in alpha. (The extinction ratio is defined as the ratio of transmitted power in the ON and OFF states of the modulator and is typically expressed in dB). Wide wells can degrade modulator performance, specifically tailored strains can be difficult to grow and, in the case of an integrated laser structure employing a common multi-quantum well growth step, can compromise laser performance.
Stepped barrier quantum wells have been proposed for EAMs. (See Shin et al, “High-Power Electroabsorption Modulator Using Intra-Step-Barrier Quantum Wells”, Journal of Applied Physics, Vol. 89, pp 1515-1517 (Jan. 15, 2001).) However, the structure was employed for analog devices to provide increased differential power transmission with respect to applied bias. It considered only the effect of the device bias on modulator absorption. It did not examine modulator refractive index effects and made no attempt to study device chirp or the small-signal alpha parameter. Reduction of the alpha parameter was not a consideration.
It is desirable, therefore, to establish quantum well EAM designs that are tailored to reduce the alpha parameter over the operating bias range of an electroabsorption modulator operating at low biases without comprising other beneficial device characteristics such as insertion loss and modulator extinction ratio.